1. Field of the Invention
The invention relates to cathode-ray tubes and, more particularly, to a device for the automatic modification of the cut-off voltage of a cathode-ray tube as a function of the luminance measured on the screen of the tube.
2. Description of the Prior Art
A cathode-ray tube 10 (FIG. 1) comprises in a chamber 11 under vacuum: a cathode 12 comprising a heated filament 16 that emits electrons and an anode 13 that is taken by means of a terminal 19 to a positive potential (HT) higher than the potential VK of the cathode so as to attract the electrons towards a surface 14 which constitutes the screen of the cathode-ray tube. The internal wall of the screen is coated with luminophores which get illuminated when they receive the electrons emitted by the cathode. This enables luminous FIGS. to be made to appear on the external wall of the screen by deflecting the path of the electrons, notably by means of variable magnetic fields given by deflection coils 15.
In order to achieve greater control over the path of the electrons and modulate the intensity of the electron beam, the electrons emitted by the cathode 12 go through a structure constituted by three electrodes or gates G1, G2 and G3 which are at potentials appropriate to their role. It is thus that the gate G1, better known as the Wehnelt gate, is positioned in the vicinity of the cathode and is at a negative potential VG1 in relation to this cathode so that it can stop or let through electrons going towards the screen. The gate G2, which is the so-called acceleration electrode, is placed in the vicinity of the gate G1 towards the screen and is at a positive potential VG2 with respect to the cathode. Finally, the gate G3, known as the focusing gate, is placed before the deflection coils 15 and is at a positive potential VG3 with respect to the cathode.
In FIG. 1, the potentials of the different cathodes are obtained schematically by potentiometers 17, 18 and 101. The potentiometer 17 is connected between a terminal at +100 volts for example and a terminal connected to the ground. The potentiometer 18 is connected between the ground and the high voltage (HT) equal to 16 kilovolts for example. The potentiometer 101 is connected between the ground and a potential of -200 volts.
The cathode 12 is connected to the output terminal of the potentiometer 17 and its potential VK can therefore vary from 0 to +100 volts. The Wehnelt gate G1 is connected to the output terminal of the potentiometer 101 and its potential VG1 may therefore vary from 0 to -200 volts. The accelerator gate G2 is connected to a first output terminal of the potentiometer 18 and its potential VG2 may therefore vary from 0 to several thousand volts. The focusing gate G2 is connected to a second output terminal of the potentiometer 18, and its potential VG3 may therefore reach several thousand volts.
It will be understood that the intensity of the electron beam and, hence, that of the luminous dot or spot on the screen can be modulated by the modification of the voltage VGK1. To this effect, the gate G1 is biased at a voltage Vco called a cut-off voltage, and a variable modulation voltage is applied to it to obtain a variable beam electron current and hence a variable luminance of the light dot on the screen.
The cut-off voltage Vco corresponds to the difference in potentials VKG1 which is just enough to prevent the passage of electrons towards the screen.
FIG. 2 is a graph showing the variation of the cathode current Ik which corresponds substantially to the luminance of the dot on the screen, as a function of the voltage VKG1 between the cathode and the gate G1. The curve 20, which is quasi-logarithmic, shows that the current Ik is zero for VKG1=Vco and that it reaches the value Iko for VKG1=0.
To obtain a linear characteristic between the signal applied to the gate G1 and the luminance on the screen, it is necessary, firstly, to linearize the curve 20 and, secondly, to hold the cathode-ray tube at its cut-off voltage in the absence of a modulation signal. This holding is all the more critical as the tube operates at low values of luminance, which it does when the cathode-ray tube is used in a dim environment.
To guarantee the stability of the low-level luminance, it is necessary:
always to bias the tube at its cut-off voltage; PA1 to keep the voltage VKG2 stable between the cathode and the accelerator gate; PA1 to keep the heating power of the cathode stable, i.e. ensure a certain precision and stability of the voltage Vf which is applied to the heating filament 16; PA1 to keep the difference in potentials VKA between the cathode and the anode stable. PA1 during the thermo-mechanical stabilization of the electron gun, when starting the system and PA1 in the course of ageing during the life of the tube. PA1 a first step of applying, to the gate G1, a voltage greater than the cut-off voltage and for the measurement of the cathode leakage currents. The result of this measurement is subtracted from the measurement made in the second step and makes it possible to do away with the effects of the leakage currents; PA1 a second step of applying, to the tube, a low modulation voltage of a known value and for the servo-control of the potential VKG1 so as to measure a cathode current Ik which is the sum of the leakage currents measured during the first step and of a constant current Iks corresponding to the assumed value that would be generated by the desired value of modulation applied.
To resolve these problems, it has been proposed to bias the tube with voltages VKG2, Vf and VKA that are as constant as possible, but it is difficult to maintain these voltages with a precision higher than 1%.
Furthermore, the characteristics of the tube change:
The result thereof is that the bias voltages would have to be readjusted in the course of time.
To compensate for these drifts, devices have been proposed for the servo-control of the cut-off voltage of the tube by the measurement of the cathode current. This servo-control is done at regular intervals, for example during the frame flyback or retrace of the image, and its value is memorized during the next frame.
The acquisition of the servo-control value is done in two steps:
Such a method is satisfactory when the dynamic range of current is between 10 microamperes and 2 milliamperes, which corresponds to servo-control currents Iks that are appropriate when the minimum light conditions are what are known as drawing room conditions as is the case with television sets used by the general public.
When the tube is placed in a very dark environment and/or when it is very sensitive (because of the high output of the luminophores), the servo-control should be done at cathode current values far lower than one microampere. This is difficult to achieve because of the values of the insulation resistance and of the inter-electrode parasitic capacitances.
Furthermore, this prior art method does not take account of the variation of the sensitivity of the luminophores, namely their light output, in the course of time.
In the patent application filed on the same date by the Applicant, entitled: DEVICE FOR THE SERVO-CONTROL OF THE CUT-OFF VOLTAGE OF A CATHODE-RAY TUBE BY A MEASUREMENT OF LUMINANCE, a device is described wherein the screen of the tube comprises a zone, located outside the surface assigned to the operational image, that is coated with a luminophore preferably emitting in the invisible range. To this zone, there is assigned a photoelectric cell which detects the luminance of an associated surface when the electron beam of the tube is directed towards this surface by an appropriate deflection. At regular intervals, for example during the frame flyback, the tube is cut off and the luminance of the surface facing the cell is measured and compared with a desired value: the signal resulting from the comparison is used to modify the intensity of the electron beam, for example by modifying the cathode voltage in appropriate sense.
Such a device is satisfactory but calls for a precise positioning of the deflection of the electron beam and of the luminance sensor so that the surface of the luminophore excited by the beam is always facing the sensor irrespectively of the circumstances. Indeed, if this is not the case, the sensor will detect no luminance and the servo-control loop will have the effect of reducing the cut-off voltage: this would lead to an increase in the intensity of the electron beam to the extent where it could cause the destruction of the tube.